This invention relates to the temperature compensated tuned circuit art, and, in particular, to the temperature compensation of tuned circuits having parabolic frequency versus temperature characteristics.
Temperature compensating schemes for stabilizing the thermal drift characteristic of tunable circuits, especially oscillators, are well known in the art. A popular family of oscillators employ tuned elements having a parabolic, thermally induced frequency drift characteristic. The surface acoustic wave (SAW) device, as well as numerous crystals (e.g., the CT, DT, NT, No. 5.degree. X, and BT types) all display this parabolic characteristic.
Prior art attempts to temperature compensate oscillator circuits of the parabolic characteristic type, have been less than satisfactory. The most commonly used compensation technique employs a complicated switching arrangement which predeterminedly switches in various control voltages across varactor diodes, which diodes are coupled in circuit with the oscillator for controlling the frequency thereof. These "piecewise" approximations to compensate for the parabolic characteristic curve are crude and do not provide a high degree of temperature stability.